Effect of spermatozoal concentration and number on fertility of frozen equine semen.

This study examines the influence of sperm concentration and number on the fertility rates resulting from the use of frozen stallion semen. The researchers found that a higher sperm concentration tended to improve pregnancy rates, but increasing the number of motile spermatozoa used in insemination did not markedly improve these rates.

Objective and Methods

• The study aimed to explore two main facets. Firstly, it sought to observe the impact of higher concentration of spermatozoa – from 400 to 1,600 million per 0.5-mL straw – on the pregnancy rate of mares. Secondly, it aimed to discern whether increasing the insemination dose from about 320 to 800 million progressively motile spermatozoa would augment pregnancy rates.
• The experimental process involved freezing multiple semen samples from each of 5 stallions in a medium made of skim milk and egg yolk. Every semen sample was split into two, with one half frozen at 400 million cells/mL and the other half at 1,600 million cells/mL.
• Insemination doses were determined based on sperm motility after thawing and contained approximately 320 to 400 or 800 to 900 million motile spermatozoa.
• Sixty-three mares were allotted to one of four sperm treatments, which varied in sperm count and concentration, and were inseminated daily.

Results

• Post-thaw sperm motility was similar for semen frozen at both concentrations. This indicates that varying concentration levels did not detrimentally affect sperm vitality.
• Pregnancy rates for the four different treatments ranged from 15% to 40%. These cervical insemination data suggested that packaging sperm at a higher concentration was associated with increased pregnancy rates, compared to packaging at a lower concentration. Interestingly, increasing the number of motile sperm in the insemination dose did not correspondingly increase pregnancy rates.

Conclusions and Implications

• The study concluded that a single 0.5-mL straw dose containing 800 million spermatozoa should be used for insemination, with each dose containing approximately 320 million motile spermatozoa.
• However, the researchers caution that fertility trials using other freezing extenders should be conducted before generalizing this insemination dose. Thus, while the research provides useful insights into sperm concentration and number effects on fertility, it is applicable only to the specific freezing medium used in the study. Equine practitioners and reproduction scientists may need to customize strategies based on particular freezing extenders used in their processes.